1. Field of The Invention
This invention relates to an asynchronous quadrature demodulator for use in a high-speed private data transmission system, where a transmission-channel distortion, which mainly is due to a multiplex transmission using a multiplex transmission channel (hereunder sometimes referred to as a multi-path), is a dominant factor of degradation of data transmission characteristics, in an office, a building, a plant or the like or for use in a high-speed data transmission system which transmits data to or from a low-speed mobile station.
2. Description of The Related Art
With development of our society, a system capable of efficiently transmitting information as much as possible has become required in recent years. For instance, a M-ary quadrature amplitude modulation system (hereunder referred to simply as an MQAM system) can increase a quantity of transmitted information without widening transmission frequency bandwidth, and is therefore effective in making good use of frequency. Thus, conventionally, the MQAM system is employed mainly in a fixed microwave communication circuit. An example of such an MQAM system is disclosed in a work entitled Digital Microwave Communication (written by M. Kuwabara and published by Kabushiki-kaisha Kikaku Centre in 1974).
FIG. 5 illustrates the construction of a conventional demodulator of a quadrature synchronization detection type employed in an MQAM system.
Hereinafter, a conventional demodulator will be described by referring to FIG. 5. In FIG. 5, reference numeral 51 denotes a quadrature detector; 52 a reference carrier recovery circuit; and 53 a data demodulator.
As illustrated in FIG. 5, a received signal 50 is supplied through a branching point to two phase detectors which detect the received signal in synchronization with each other by respectively using two reference carriers different in phase from each other by 90 degrees. Thereafter, an I-axis output (i.e., an I-signal) 54 and an Q-axis output (i.e., a Q-signal) 55 outputted from the detector 51 as a result of the synchronous detection are inputted to the data demodulator 53 in which the inputted I and Q signals are discriminated from each other and then original modulating signals respectively corresponding thereto are recovered therefrom.
Referring now to FIG. 6, there is shown an example of the reference carrier recovery circuit 52 for issuing the reference carriers to both of the phase detector and a phase shifter of FIG. 5. The reference carrier recovery circuit of FIG. 6 is what is called a selective control type carrier synchronization circuit for use in a 16-ary quadrature amplitude modulation system (hereunder referred to simply as a 16-QAM system). As is seen from FIG. 6, this reference carrier recovery circuit selects a signal having a phase component corresponding to 4-level phase shift keying signal (hereunder referred to as a 4-level PSK signal) from a 16-QAM wave by using a signal demodulated and recovered. Thus, a phase-locked loop using only this signal is formed.
It is known to those skilled in the art that the construction of such a reference carrier recovery circuit employed in an MQAM system becomes rather complex as the example of FIG. 6 is.
Incidentally, in case of a private data transmission system used in an office, a building or the like, a portable small-sized demodulator is needed. The above described demodulator of the quadrature synchronization detection type, however, requires a reference carrier recovery circuit and thus has a complex structure. Therefore, the conventional demodulator has a drawback that it is not suitable for miniaturizing. The present invention is created to eliminate the above-mentioned drawback of the conventional demodulator.
It is accordingly an object of the present invention to provide a small-sized demodulator which has a simple structure and does not require a reference carrier recovery circuit. Further, it is another object of the present invention to provide an asynchronous quadrature demodulator which can stably operate in a transmission channel having a transmission-channel distortion due to a multiplex channel.